1. Field of the Invention
The present invention relates to a femoral implant used as a hip-joint prosthesis. In particular, the invention relates to implants made of composite laminates of continuous fiber in a matrix. For example, the fibers may be carbon, boron, ceramic, metal, aramid fibers (e.g. Kevlar), or fiberglass, and the matrix may be biocompatible a polymer, metal, ceramic, or carbon.
2. Discussion of Background Information
A femoral implant, as the name implies, replaces the end of a femur in a hip-joint prosthetic device. The femoral implant basically includes a longitudinal stem or shaft that is connected to bone. In present practice, the stem sits in a cavity formed in the proximal region of the femur. A neck extends from the shaft terminating in a ball, which cooperates with the acetabulum, or socket, of the hip joint in the pelvis. To insert the implant, the head of the femur is removed and a cavity formed in the bone just below the cut. The shaft of the implant is then anchored into the cavity using, e.g., a press-fit or bone cement.
Implants made of fiber embedded in a polymeric matrix have been used in place of earlier metal implant designs. Fiber-matrix composite implants can be engineered to exhibit structural properties more closely resembling that of natural bone, which has less of an adverse effect than implants much stiffer than natural bone.
One method of making an implant of continuous unidirectional fiber involved stacking layers having parallel conditions unidirectional fibers in a matrix, in which the orientation of the fiber in each layer was arranged in a parallel manner. The orientation of the fiber in the final implant could then be varied by stacking the individual layers in such a way that the fibers were aligned in the desired direction. The final product was produced by heating the matrix in which the fibers were embedded in order to cause the matrix to flow. Upon cooling, the matrix hardened into a composite block in which the various layers of fiber were aligned as desired.
Fibers were aligned in these composite blocks in directions wherein increased strength was considered to provide optimum results. For example, reinforcement was provided along the shaft, i.e., the longitudinal axis, by orienting a majority of the fibers in that direction. Reinforcement was also provided by orienting fibers at an acute angle to the longitudinal direction, balanced by fibers oriented in the negative acute angle direction, producing a mirror image about a sagittal plane of the device. However, it was believed that the shaft region should be more strongly reinforced than the neck region. For example, see U.S. Pat. No. 4,892,552 and "Carbon Materials For Endoprosthetic Joints", K.J. Huttinger and W. Huettner, Extended Abstracts of the International Symposium on Carbon, 1982, pages 138-149, the disclosures of which are incorporated by reference in their entireties.
Also, U.S. Pat. No. 5,064,439, the disclosure of which is hereby incorporated by reference in its entirety, discloses a load-bearing prosthetic device, such as a hip stem with a longitudinally curved body. The prosthetic device is made from continuous filament fiber plies with parallel oriented fibers in each ply. The plies are curved longitudinally to correspond to the shape of the body. In one embodiment, the plies at or near the surfaces have longitudinally oriented fibers and the plies between the surface layers have fibers offset at 5.degree.-40.degree. from the longitudinal axis. The fiber orientation is balanced by providing a ply of negatively angled offset fibers for each positively angled offset ply. Table III of U.S. Pat. No. 5,064,439 shows examples of unbalanced fiber orientations. None of the examples, however, teach a fiber orientation wherein at least 50% of the layers have fibers oriented in the .theta. direction and the remainder of the layers have fibers oriented in directions other than the .theta. angle, where .theta. is the acute angle formed between the longitudinal direction of the shaft and the neck extending therefrom.
A femoral implant made from layers of fiber in a polymeric matrix is disclosed in U.S. Pat. No. 5,163,962, the disclosure of which is hereby incorporated by reference in its entirety. The femoral implant has a longitudinal shaft having a neck extending therefrom at an acute angle .theta. to the longitudinal direction. The layers of fibers are arranged such that they are balanced with at least 50% of the layers in the .+-..theta. directions.